Recently, with the concerns about depletion of fossil resources as major energy sources and environmental problems such as the greenhouse effect caused by carbon dioxide emission resulting from combustion of the fossil resources, the importance of development of environment-friendly alternative energy is increasing. In an effort to overcome these problems, various energy sources including hydraulic and wind power are being studied. Also, the solar light is being studied as a new renewable energy source that can be used unlimitedly. A photovoltaic cell using the solar light can be largely classified into a photovoltaic cell using an inorganic material such as silicon and one using an organic material. Especially, a polymer-based organic thin-film photovoltaic cell is studied a lot for many advantages over a silicon-based inorganic photovoltaic cell, including low production cost, lightweightness, production by various methods including roll-to-roll processing and inkjet printing and production of large-sized flexible devices that can be bent freely. Typically, poly(3-hexylthiophene) (P3HT) is used for a photoconversion layer of an organic thin-film photovoltaic cell. When a device is fabricated using the compound together with a C61 fullerene derivative having high electron affinity, it is reported that an efficiency of about 4-5% is achieved. However, the optical absorption wavelength of P3HT is limited to about 650 nm (G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nat. Mater., 2005, 4, 864-868; W. L. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct. Mater., 2005, 15, 1617-1622).
In order to overcome these problems and to fabricate a high-efficiency organic photovoltaic cell, development of a new low band gap polymer which exhibits a broad optical absorption wavelength, superior hole mobility and adequate molecular energy level is necessary.